Electric wave amplifying system



July 25, 1939. s. T. MEYERS 2,157,357

' ELECTRIC WAVE AMPLIFYING SYSTEH Filed Dec. 5, 1956 2 Sheets-Sheet 1 FIG. 2

INVENTOR $.71 MEVERS ATTORNEY 5. T. MEYERS ELECTRIC WAVE AMPLIFYING SYSTEM July 25, 1939.

Filed Dec.

5, 1936 Z Sheets-Sheet 2 INVENTOR s. r MEVERS ATTORNEY Patented July 25, 1939 PATENT OFFICE 1 2,167,367 ELECTRIC WAVE AIVIPLIFYING SYSTEM Stanley T. Meyers, East Orange, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application December 5, 1936, Serial No. 114,400 4 Claims. (01.179-171) vacuum tube type, the feedback coil causes little This invention relates to'wave amplifying systems.

Objects of the invention are to control transmission properties of such systems, as for ex- 5 ample, transmission efiiciency, distortion, feedback, and impedances, and relations of such properties in such systems.

It is also an object of the invention to provide simple systems affording such control.

In one specific aspect the invention is a system comprising a simple amplifier which has negative or gain-reducing feedbacks that reduce distortion and render the insertion gain of the amplifier independent of frequency.

The feedbacks may be, for example, a seriesseries negative feedback anda series-shuntnegative feedback. (A' series-shunt feedback "refers to a feedback that is series feedback at the amplifier input and shunt feedback at the amplifier output; and a: series-series feedback refers to a feedback that is series feedback at the amplifier input and series feedback at the amplifier output.) I 7 a.

The series-series feedback may be obtained, for example, by a feedback. resistor or impedance in series with the secondary winding of. an input transformer for the amplifier and also in. series with the primary winding of an output transe former for the amplifier; and the series-shunt feedback may be obtained, for example, by a feedback coil in series With the secondary winding of the input transformer and inductively related to the wi dings of the output transformer. W

Then the feedback path, including the feedback resistor and the-feedback coil in series with each other, is in series with" the secondary winding of the amplifier input transformer, and therefore the impedance of; the feedback path as 40: viewed from its terminals at the input side of the amplifier (i. 'e. "as viewed from the cathode and the low voltage end of the secondary Winding of the input transformer) can be low without introducing undue loss in' transmission from the incoming line to the am'plifien] Having this impedance low minimizes deleterious effects of distributed shunt capacity in the feedback circuit upon phase shift invthe feedback path and singing tendency in the amplifier, A1s o, due tothis low' impedance the tendency for. longitudinal voltages from the primary winding of "the input transformer to appear across the feedback path and thus be amplified and transmitted to the erable amounts of (series-series and series-shunt) negative feedback, by proportioning the amplioutput is reduced] When the amplifier for iexample br ne created for example by space current of th 1 tube flowing through the resistor.

If desired, the amplifier input and output impedances ZA and ZB can be resistance impedances, with ZA and Z3 each independent of variations in the attached impedances Z1 and Z2; and

moreover, if desired, ZA can be a match for Z1.

- With large values of negative feedback of both types (i. e., series-series and series-shunt), the insertion'gain of the amplifier can be made independent of, frequency and ZB is practically independent of. variations in and R0 or of variations in the amount of feedback, and by adjustment of the amplifier circuit, as described hereinafter, the gain of the amplifier can be changed without causing variation of ZB of the amplifying element or path of the amplifier; and the quantity R0 is the output resistance 'or impedance of that path, as for example, the

plate-cathode resistance in the last tube in the case of a vacuum tube amplifier.

Regardless of the amount of feedback in the amplifier, the feedback is practically independent of. variations in Z1. This constancy of the The 1 quantity, referred to here is the amplification feedback not only tends to give constancy of the distortion reduction and gain stability produced by feedback, but is of especial value for avoiding singing around the feedback loop, 1. e., for stabilizing the amplifier against singing or for reducing singing tendency.

Also regardless of the amount of feedback in the amplifier, the amplifier circuit can be proportioned as described hereinafter to render the feedback independent-0f Z2 to any desired degree, and this can be done Without necessitating connction of loss-producing resistance across the path from Re to Z2; but if complete independence is to be obtained and at the same time the insertion gain-of the amplifier is to be made independent of frequency as referred to above, then Z1 andv Z2 must be resistance impedances.

For given values of Z1 and Z2 and with considfier as described hereinafter, ZA can be made equal to Z2 with ZB equal to Z1; but then the insertion gain of the amplifier, in general will not be independent of frequency, though the independence can be obtained if Z1 and Z2 be made resistance impedances.

Other objects and aspects of the invention will be apparent from the following description and claims.

Fig. 1 is a circuit diagram for facilitating explanation of the invention, showing a wave amplifying system claimed in divisional application Serial No. 169,116, allowed January 14, 1939, employing shunt-series and series-shunt feedbacks;

Fig. 2 is a circuit diagram of a wave amplifying system embodying a form of the invention, employing series-series and series-shunt feedbacks; and

Figs. 2A and 2B show modifications of the system of Fig. 2. V

The system of Fig. 1 comprises an amplifier whose amplifying path may be, for example, an amplifying element 1 of the vacuum tube type having a single vacuum tube stage or any desired number of tandem connected stages, G and P designating the grid of the first tube and the plate of the last tube.

The amplifier may be, for example, a stabilized feedback amplifier of the general type in which a portion of the output wave is fed back in gainreducing phase and in amount suificient to reduce distortion below the distortion level without feedback. Such feedback is disclosed, for example, in H. S. Black Patent No. 2,102,671, December 21, 1937, for Wave translation system, and in the article by Black on Stabilized Feedback Amplifiers published in Electrical Engineering, January 1934, pages 114 to 120.

The amplifier of Fig. 1 has an input transformer 2 with a primary winding m1 of mi turns and a secondary winding msofma turns, and has an output transformer 5 with a primary Winding m1 of in turns and a secondary winding m of m turns.

Waves to be amplified by the amplifier, as, for example, speech current waves or speech modulated carrier waves, are supplied from an incoming line or circuit Z1, of impedance value Z1 comprising a wave source E of voltage E. The amplified waves are transmitted to outgoing line or circuit Z2 of impedance value Z2.

The amplifier comprises two feedback paths or circuits 6 and I. Path 6 comprises a feedback coil m2 of mg active turns on transformer 2. This coil m2 is in series with winding m, and provides shunt-series negative feedback. Any parasitic series-series feedback which it produces does not materiallyaffect the operation of the circuit provided such feedback is small compared to the shunt-series and series-shuntfeedbacks. Path I comprises a feedback coil m of 112 active turns on transformer 5. This coil n2 is in series with winding m3, and provides series-shunt negative feedback. A C-battery 8 for providing negative grid bias is shown in the lead from the lower end of coil m. A B-battery 9 is shown for supplying space current. If desired, any series-series feedback produced by the impedance of coil m2 can be avoided by shifting the connection of the positive terminal of battery8 to the cathode.

ar na] The amplifying path of the amplifier may be referred to as the ,u-CiI'CLlit, and the feedback circuits may be referred to as the B-circuits, or the fii-circuit 6 and the [ea-circuit 1, the significance of ,u. and ,8 being as indicated in the Black patent and article mentioned above. The feedback through path 6 may be, for example, negative feedback with ,u.,81 1 and the feedback through path 1 may be, for example, negative feedback with ,ufl2 1. Preferably the coupling between m2 and mi, and also the coupling between 112 and n3, is as close as practicable. The closer this coupling, the more nearly complete will be the correction that the feedback produces in the transformer distortion, (e. g., themodulation produced in the transformers and the distortion due to variation of the transmission efiiciency of the transformers with frequency.)

With an amplifier of any number of stages and input and output transformers with three windings as shown, the gain and input and output im pedances may be made to have the values indicated by the following formulae.

It can be shown that the input impedance with feedback is u 2 2?) where R0 is the plate impedance in the last tube.

With considerable amounts of feedback, or in other words if a be made large, so that all terms involving ,u. are large compared to all other terms in the equation, the equation may be simplified and the input impedance becomes:

ance is:

2 R.,+Z1( [1+#(1+EE\)] e I13 n Then, as before, with considerable amounts of feedback, or when ,u. is large so that terms in which it is involved are large compared to all other terms in the. equation, ZB becomes arm-a The factor involving the turns ratios is the inverse of that in (1A) and is unity when that a in (1A) is unity, making ZB=Z1. (2-13) The voltage amplification, which is the ratio of the voltage V across Z2 to the voltage E can be shown to be 25mg) si e-a1- if the feedback is large (3) simplifies to m s 1 (3A) ITK EXITS) n n 2 m2 2 s CIXQ) +21 21) 5) Here again under the conditions of the turns ratios above,

2 Y a) 2) E- 2+ 1 The transmission is then times the transmission before the amplifier was inserted. Thus, the insertion gain of the amplifier is then constant or independent of frequency.

Fig. 2 shows a modification of the circuit of Fig. 1 in which an impedance KRo, giving seriesseries-negative feedback through a feedback path [1, replaces the feedback coil ma that gave shuntseries feedback through feedback path 6 of Fig. 1. The factor K may have any desired value, constant or variable, and if desired may be a complex quantity. Thus, the impedance KRo may be given any value desired.

The amplifier output impedance in Fig. 2 can be obtained directly from Equation 2 by substituting KRo for Z1 and (since the coupling between m2 and ma has been eliminated) dropping out the term It is From similar treatment of Equation 3, i. e., by substituting KRo for Z1 and eliminating all components of propagation due to any coupling between m2 and 1m or ms, the voltage amplification is found to be:

If the feedback is made large (i. e., if terms involving a are large compared to all others), Equation 7 becomes Equation 9 shows that the voltage amplification is a constant times the impedance ratio 2 Z2+ZB' Equation '7A shows that Zn can be made equal to Z1 by making KR equal to If this be done, Equation 9 shows that the voltage amplification will become or in other words that the voltage amplification will be a constant B31113 (are) times the voltage across Z2 working out of Z1 before the amplifier was inserted. Thus the insertion gain of the amplifier is then constant or independent of frequency.

However, though the insertion gain is then constant, a large reflection may occur at the amplifier input since the self impedance of the input transformer and consequently the value of ZA ordinarily is high compared to the value of Z1. Where this reflection is not desired, it can be avoided by connecting across the winding m1 or me a terminating impedance of proper Value to make ZA match Z1. For example, generalized impedance D of value D may be connected across winding m1 by switch [8, and D may be equal to Z1 Where the impedance presented by winding m1 is so high in comparison to D that the value of ZA is substantially D. The connection of D modifies by a factor the value of amplification given by equation 9, but the insertion gain of the amplifier will still be constant or independent of frequency if the factor is constant or independent of frequency, and the value of this factor is 1/2 when D is equal to Z1.

When desired, the amplifier input and output impedances ZA and ZB can be made resistance impedances. With the self impedance of the input transformer high, ZA is equal to D, and so can be made a resistance impedance of any chosen value by making D a resistance impedance of the chosen value. As shown by Equation 7, ZB can be made a resistance impedance by making KRo a resistance impedance; and, with either ,a or

the feedback made large, equation 7A shows ZB is equal to KRo times the turns ratio.

When KRo has a resistance component, the

steady voltage drop created across the resistance by the flow of space current from source 9 can be used as a grid biasing voltage.

As indicated above, the impedance of the feedback circuit to which the input connection is made, i. e., the impedance of the feedback circuit through paths I1 and l in series as viewed from the cathode and the lower end of winding me, can readily be made low, minimizing deleterious effects of distributed capacity of the feedback circuit. The loss in transmission from Re to Z2 can readily be made low.

ZA is substantially independent of variations in Z2 and feedback.

ZB and e are each practically independent of variations in both Z1 and D. This independence is obtained by having the feedbacks series feedbacks at the amplifier input, inasmuch as the grid-cathode impedance in tube I is high come pared to the impedance presented by winding m3. ple, in facilitating maintaining Zn matchedto Z2; and this constancy of ,u,8 is of value for example in stabilizing against singing. The variations in Z1 might be due for example to variations of the impedance of the incoming line with fre 1. e., by making s can be made independent of Z2 or any desired approximation or closeness of approach to the independence can be obtained by sufficiently closely approximating the relation However, if complete independence is to be obtained and at the same time the insertion gain of the amplifier is to be made independent of frequency as described above, then Z1 and Z2 must be resistance impedances;

For given values of Z1 and Z2 and with considerable amounts of negative feedback, ZA can be made equal to Z2 with ZB equal to Z1, by making D equal to Z1 and making KRO equal to but then the insertion gain of the amplifier in general will not be independent of frequency, though the independence can be obtained if Z1 and Z2 be made resistance impedances.

As shown by Equation '7, ZB is independent of Z2.

As shown by Equation '7A, with considerable amounts of negative feedback, ZB is constant when KRO times the turns ratio is maintained constant. By varying these two factors (i. e. KRD and this turns ratio) inversely, so their product will not vary, the amplifier gain can be changed without causing variation of ZB. For example, with a given proportion of seriesseries and series-shunt feedbacks, m can be increased by a given percentage and KRo increased by the same percentage, to change the amount of feedback without varying ZB. This increases the negative feedback in the amplifier (and decreases the amplifier gain).

Fig. 2A shows a modification of Fig. 2 in which a single adjustment, instead of the two adjustments provided at KRO and m in Fig. 2, serves to change the gain without causing change of ZB. In Fig. 2A a potentiometer resistance I'i is connected across KRo and 112 in series, and winding m3 is connected to a contact I9 movable along this resistance, (insteadof to a contact movable This constancy of Zia-is of value; for eXa-m-- along mustthat by adjusting the contact I9 the amplifier gain can be varied without causing variation of ZB.

Fig. 2B showsa two-Way repeater for a circuit 20 which may be, for example, a carrier telephone circuit. The repeater comprises an east amplifier for amplifying waves transmitted from west to east and a west amplifier for amplifying waves transmitted from east to west. Each amplifier is'similar to theamplifier of Fig. 2. The terminating resistance D, across winding m3, corresponds to resistance D across winding m1 of Fig. 2.

Transmission from west to east may be, for example, in the frequency range above 7,150 cycles per second andtransmission' from east to west may be, for example, in the frequency range below 7,150 cycles per second. The waves to be amplified by the east amplifier are selected by directiona'l' high-pass filter 2I and transmitted through variable attenuator 22 to the amplifier. The attenuator may be used to adjust the gain of the amplifier circuit. The amplified waves pass through directional high-pass filter 23' to line 20 for transmission to' the east,

Waves to be amplified by the west amplifier are selected by directional low-pass filter 24 and transmittedthrough: attenuator 25', the west amplifier and directional low-pass filter 26.

When switch 21 is raised, filament heating current for the tubes I is supplied from the lower section of battery 9 and space current for the tubes is supp-lied from the whole of the battery. The filament heating current passes from the battery through conductor 28, ballast lamp 29, the filament of the east amplifier, the filament of the west amplifier, conductor 33, and the lower spring of the switch. The space current passes from the battery through the top spring of the switch, conductor 30, windings n1, tubes I and resistances KRo, conductor 3%, the middle spring of the switch, conductor 32 and the lower spring of the switch. A filtering network 34 for the space current supply circuit comprises a capacity and a resistance which are shunted across conductors 30 and 3I and whose Values may be, for example, 4 microfarads' and one-tenth megohm.

When switch 2'! is lowered, filament heating current for the tubes I is supplied from a secondary winding 35 of transformer 36, whose primary winding is. supplied with current from a GO-cycle source of power 31, and space current for the tubes I is supp-lied from a rectifier 40 operated from transformer 36. The filament heating current passes from winding 35 through the ballast lamp 29, the filaments of tubes I in series, conductor 33, the lower spring of switch 21, and conductor ll. The space current for tubes I passes from the filament of the rectifier G0 through a resistance 44, the top spring of switch 21, windings n1, tubes I and resistances KRo, conductor 3|, the middle spring of switch 21, conductor and winding 46 of transformer 36. The resistance 44 may havea value of 4500 ohms, for example. This resistance controls the value of the direct currentsupplied' to the tubes I, and alsocooperates with the filtering network 34 to form a resistance-capacity filter for the plate current supply circuit.

The feedback resistors KRo supply the grid biasing .potentials for the control grids of the tubes I. I

Steady potentials for the screen grids of the tubes I are supplied from the place current supply conductors 30 and 3I through resistors 48,

serially related to said load circuit with respect to said output circuit, and means effectively conmeeting a portion of said load circuit in serial relation to said input circuit with respect to saidv wave source and thereby supplying to said input circuit a voltage in series withthat of said source and dependent upon that across said load circuit.

2. An amplifier with input and output transformers each having a primary winding and a secondary winding, a feedback impedance in serial relation to said secondary winding of said input transformer and also in serial relation to said primary winding of said output transformer, and a feedback coil in serial relation to said secondary winding of said input transformer and in- Y formers each having a primary winding and a secondary winding, means for producing negative feedback in said amplifier, comprising a resistance in serial relation to said secondary winding of said input transformer and also in serial relation to said primary winding of said output transformer, and a coil for producing negative feedback in' said amplifier, said coil being in serial relation to said secondary winding of said input transformer and forming a third winding of said output transformer.

4. A system comprising an amplifier including a vacuum tube with an input transformer and an output transformer, a sending impedance from which said input transformer works including a source of speech modulated carrier waves, a receiving impedance into which said output transformer works, a source of direct current for supplying space current to said tube; a feedback resistance in serial relation to said secondary winding of said input transformer and also in serial relation to said primary winding of said output transformer, a feedback coil for producing negative feedback in said amplifier, said coil being in serial relation to said secondary winding of said input transformer and forming a third winding of said output transformer and a terminating resistance in shunt relation to one of said windings of said input transformer, said terminating resistance having its magnitude such'as to make the impedance facing said sending impedance substantially match said sending impedance, said feedback resistance having its magnitude such as substantially to match the magnitude of the amplifier output impedance to the magnitude of said receiving impedance, and said feedback resistance being connected between the cathode of said tube and the negative terminal of said space current supply source, and said source being connected between one terminal of said coil and the anode of said tube. 

